The diffusion spectrometry of X rays is based on exposing a material to incident X rays with energy equal to a few tens to a few hundreds keV. When they encounter the material on which they are projected, the X photons induce different types of interaction with the material: fluorescence or internal conversion (photoelectric effect during which the photon transfers all its energy to the material which returns it afterwards), inelastic diffusion, (or Compton effect which includes a change in the direction of the photon and a reduction of its energy), creation of positron-electron pairs (uniquely for X rays with very high energy not considered in the present invention), or Rayleigh diffusion (or elastic diffusion, a minority of the considered energies).
The invention uses electromagnetic radiation sources with energy between 0 and a few hundreds keV, for instance 300 keV. It can involve X ray generating tubes. Beyond 30 keV the Compton diffusion phenomenon is predominant for organic materials.
Certain characteristics of the studied materials (linear attenuation coefficient μ (E), density, ratio
  Z  Abetween the atomic number Z and the atomic mass number A) can in principle be determined on the basis of theoretical knowledge and by obtaining the primary diffuse spectrum of the material exposed to X rays, in other words, the diffuse radiation spectrum which is obtained in a situation whereby each photon interacts only once with the material. The X diffusion spectra comprise an important component of diffuse photons that have interacted several times with the material. This component is called the multiple diffuse radiation spectrum.
Certain information, in particular the density of the material, can be obtained based on the total diffuse radiation spectrum because the attenuating character of the material affects in the same manner the two components, primary and multiple, of the diffuse radiation spectrum. To obtain a better estimate of this density, and other physical and chemical information, it is known that the use of the total diffuse radiation spectrum leads to imperfect results.
PCT Publication No. WO2007/007247 discloses the use of transmission data to determine the multiple diffuse radiation component.
The invention is placed in another context, because it applies in the first place to an analysis system with a strongly collimated radiation source, and a detector placed in such manner that it collects diffuse radiation.
This detector can be placed in the same half-space as the source opposite the surface of the material studied, and also strongly collimated. This is called a retro-diffusion configuration. The detector can also be placed in such manner that the studied material is situated between the source and the detector. This is called a diffusion configuration. In the last case, the collimation of the source and/or the detector avoids that the radiation transmitted through the material is not detected (in particular, not having interacted with the material).
U.S. Pat. No. 6,320,933 discusses an analysis of the diffusion of retro-diffuse X rays. The ratio of retro-diffuse intensities measured by a detector detecting all radiations and a detector measuring only the multiple diffuse radiation component gives only an estimate of the density. On the contrary, thanks to a specific processing algorithm, the invention described below distinguishes between the multiple diffuse radiation and the primary diffuse radiation and provides more numerous physical and chemical information.